FIG. 1 shows a conventional charging system 100 which includes a charger 102 for charging a battery pack 104. The charger 102 includes an AC/DC (alternating current/direct current) converter 108 that converts an input AC power to an output DC power to charge the battery pack 104 under control of a charger controller 110. The battery pack 104 includes a primary protection circuit 118 that controls a first switch 114 and a secondary protection circuit 120 that controls a second switch 116. During the charging period, both switches 114 and 116 are on. When a cell voltage of the battery cell 122 reaches a first threshold VCV, the primary protection circuit turns off the switch 114 to stop the charging. In case the primary protection circuit 118 fails to perform the desired protection, for example if the cell voltage keeps rising, the secondary protection circuit 120 can detect this condition and turn off the switch 116 when the cell voltage reaches a second threshold VOV.
In some low cost applications or some applications with very limited printed circuit board (PCB) space such as electric toothbrushes, the protection switches 114 and 116 and corresponding protection circuits 118 and 120 not only increase the total cost, but also increase the space requirement. In order to reduce the cost and PCB space for these compact and low cost applications, a new solution is needed.